In a wireless communication system, such as a cellular system, a reference signal is introduced to obtain various indexes of a propagation channel and a transmission signal. For example, LTE (Long Term Evolution) of a next-generation communication system which is studied in the 3GPP (3rd Generation Partnership Project) as the international standards organization of mobile communication uses a reference signal (RS). In downlink communication from a base station to a user equipment, a reference signal which is transmitted from a transmission apparatus (base station) to a reception apparatus (user equipment) is mainly used for (1) estimation of a propagation channel for demodulation, (2) quality measurement for frequency scheduling and adaptive MCS (Modulation and Coding Scheme) control, and the like. In the LTE, a reference signal is transmitted in a predetermined unit of wireless resources in a multi-antenna system for applying MIMO (Multiple Input Multiple Output).
In LTE-advanced (hereinafter, referred to as LTE-A) which is a more advanced communication system than LTE, in order to achieve high-speed performance, the introduction of high-order MIMO (for example, eight transmission antennas), coordinated multiple-point transmission and reception (CoMP), or the like is studied. For this reason, in addition to a reference signal (first reference signal) which is studied in LTE, an additional reference signal (second reference signal) for LTE-A is needed, and a transmission method thereof is discussed.
For example, as described in Non-Patent Literature 1, in LTE-A, two types of reference signals are studied for the above-described purposes.
(1) Demodulation RS: for PDSCH (Physical Downlink Shared Channel) demodulation, and specific to user equipment (UE) (UE-specific) with application of the number of layers same as PDSCH and precoding.
(2) CSI-RS: for CSI (Channel State Information) measurement, (examples of CSI include CQI (Channel Quality Indicator), PMI (Precoding Matrix Indicator), RI (Rank Indicator), channel matrix, channel covariance matrix, interference component, and the like), and specific to a cell (cell-specific) with no application of precoding. Examples of specific channel quality information include CQI corresponding to a combination of a predefined modulation scheme and code rate, PMI which selects a precoding matrix based on a current channel condition from a predefined codebook, RI corresponding to the desired number of transmission links, a channel matrix in which the fading value of a MIMO channel is expressed in a matrix, a channel covariance matrix in which the components of the channel matrix are used in a channel covariance matrix, an interference component obtained by subtracting a desired signal from a reception signal, and the like.
However, the purposes are not exclusively positioned. Specifically, a discussion proceeds assuming that the CSI-RS may be used for the purpose (1).
In LTE, the minimum unit of frequency scheduling and adaptive MCS control is defined as a resource block (RB, and hereinafter, referred to as RB) in a frequency domain and as a subframe in a time domain. The signal configuration of one subframe or RB (hereinafter, referred to as 1 RB/subframe) as a resource unit is made such that a control signal and a reference signal RS are allocated from the head of the time axis, and data is subsequently allocated. The reference signal RS is allocated in a specific OFDM symbol and a specific subcarrier of the 1 RB/subframe. As an example of the CSI-RS transmission method for LTE-A, a method is known in which a CSI-RS (second reference signal) for 8 antennas is transmitted only in a specific RB/subframe, and a 4 antennas-compliant 4RS (first reference signal) for LTE is transmitted in another RB/subframe (for example, see Non-Patent Literature 2).
In the CSI-RS transmission method, a configuration is made such that an LTE user equipment compatible with only LTE can receive data at a resource which does not transmit the CSI-RS, and the 4RS for LTE is transmitted at a resource of a specific RB/subframe, allowing the LTE user equipment to perform CSI measurement. Since the RB/subframe in which the CSI-RS for 8 antennas is transmitted is arranged discretely, it is possible to perform CSI measurement with satisfactory accuracy at each resource by interpolation/averaging between the resources.
The need for discontinuously transmitting a CSI-RS as a reference signal in the above-described manner will be described. CSI-RS transmission significantly adversely affects an LTE user equipment compatible only with an existing system. Specifically, if a rule is provided to multiplex a resource transmitting a CSI-RS to only an allocated resource of an advanced system-compliant user equipment (hereinafter, referred to as an LTE-A user equipment), there is a restriction on scheduling with respect to the LTE user equipment. If such a rule is not provided, a signal which cannot be recognized from the LTE user equipment on the LTE user equipment-allocated resource is multiplexed, causing degradation of demodulation performance in the user equipment. Since any phenomenon described above is not preferable, a solution is made such that CSI-RS transmission is limited at a specific time resource focusing on resources in the time domain. That is, it is necessary to discontinuously transmit the CSI-RS, instead of transmitting the CSI-RS in continuous subframes.
An example of a procedure of a channel quality information (CSI) request, CSI measurement and report will be described with reference to FIG. 30. FIG. 30 is an operation explanatory diagram showing a procedure of a channel quality information request and a report in response to the channel quality information request in LTE. Here, description will be provided assuming communication between a base station (eNB: evolved Node-B) and a user equipment (UE) in a cellular mobile communication system.
The base station (eNB) transmits a cell-specific reference signal (cell-specific RS: CRS) in each subframe. When an instruction for a CSI request is indicated to a user equipment (UE1 or UE2), the base station sends a notification to the user equipments using a downlink control channel PDCCH (Physical Downlink Control Channel). Here, as an operation example of the user equipment, the behavior of a first user equipment UE1 will be described. If an own apparatus-addressed CSI request is detected by the PDCCH, UE1 performs CSI measurement using the CRS of the subframe, and reports CSI to the base station using an uplink data channel PUSCH (Physical Uplink Shared Channel) after a predefined number of subframes (in this case, 4 subframes). The base station performs uplink resource allocation assuming a PUSCH subframe for a CSI report from the user equipment, and indicates a request by the PDCCH. In the base station, an uplink signal from UE1 is received in accordance with the PDCCH which indicates the request and the content of the CSI report is detected. The base station realizes frequency scheduling of downlink data and adaptive MCS control using the obtained CSI.
The reasons for defining the number of subframes from the CSI request to the CSI report include a reason from the viewpoint of the CSI report due to the amount of processing in the user equipment, and a reason from the viewpoint of the utilization of CSI when the scheduler of the base station which receives the CSI report utilizes the relevant information. For the former reason due to the amount of processing in the user equipment, it is necessary that CSI measurement is performed after the CSI request is received, the amount of processing necessary for encoding/modulating the relevant information to generate a signal is taken into consideration, and a processing time equal to or greater than a given time is provided for realization with no large load. Accordingly, it is necessary to define the number of subframes equal to or greater than a given time as a requirement from the viewpoint of the CSI report.
For the latter reason from the viewpoint of the utilization of CSI, processing is performed for allocating the resource of each user equipment on the basis of the reported CSI information. Thus, taking into consideration the efficiency of resource allocation, it is preferable that there is a slight change between the actual allocation time and the CSI measurement time. However, in a wireless propagation environment, as the time elapses, the change in CSI increases due to time-dependent fading. For this reason, as the requirement from the viewpoint of the utilization of CSI, it is necessary to set a subframe within a given time in which the efficiency of resource allocation is not significantly degraded. The number of subframes from the CSI request to the CSI report is determined such that the requirements from the two viewpoints are compatible.
The same discussion is established as to a reference signal at the time of uplink communication from a user equipment to a base station. In LTE-A, the introduction of a technique, such as MIMO, is studied in which multiple transmission systems (antennas and transmission amplifiers) are provided in a user equipment, and it is necessary to transmit a reference signal for frequency scheduling and adaptive MCS control using multiple transmission systems. Thus, the expansion of a sounding RS (SRS) which is one of the reference signals for use in LTE in compliant with multiple transmission systems is studied. For example, as described in Non-Patent Literature 3, a method is studied in which an SRS is transmitted from a user equipment at an indicated timing in accordance with a request from a base station. With regard to SRS transmission in LTE, similarly to the need for discontinuous transmission in CSI-RS, the SRS can be transmitted and received once at a set interval Tsfc of the base station LTE such that an adverse effect on data transmission of the user equipment is minimized.